The AM Forum

I 'm new here.  My name is Charles and I have been a HAM since 1959.  I even have a commercial radiotelephone license.  That only means I have probably answered all the FCC questions about AM correctly.

' In my head are many facts T
hat, as a student, I have studied to procure,
In my head are many facts..
Of which I wish I was more certain I was sure! '

(King of Siam - The King and I)

When I question the details, I really don't understand how AM works .

I have heard AM modulation described as a process during which the amplitude of a carrier signal is varied in accordance with the amplitude of an information waveform.  This is not so.

A look at the equation describing the spectrum of an AM signal shows the three distinct components.  The origional carrier, with no variations in amplitude due to the modulation and the two sidebands created from the modulating power.

My first hurdle to overcome is the AM scope pattern.  Why does it show zero amplitude on the negative peaks of a 100% modulated signal when a carrier of nonvarying amplitude is continuously present?  I'm not sure if I'm correct, but here is my explaination:  

The AM scope pattern vector sum of the three components.  Since the sidebands are frequencies offset from the carrier, they are continuously varying in phase with respect to it and the additions and subtractions result in a pattern that represents some kind of envelope.

This is a hurdle that I haven't been able to rationalize:

The ARRL handbook shows the typical transformer isolated plate modulation circuit.  It states that for 100% modulation, modulation voltage developed accross the trasnsformer secondary will cause the final amplifier plate voltage will vary between zero and twice the B+ voltage.  That seems to make sense in the context of a carrier amplitude variation scheme.  But, how can the amplifier continue to deliver the always present carrier power when its plate voltage is zero?

Maybe there is energy stored in the tank circuit or something nonintuitive like that going on.

Tis a puzzlement!

---  CHAS  WA2DYA

The final amplifier is a mixer producing 2 side bands with the carrier and on a scope you know why you see the voltage change. Now fire up your spectrum analyzer and observe the carrier with the two side bands 6 db down from carrier at 100% modulation. That is why it takes 1/2 as much audio as carrier to produce 100% modulation.  !/4 power in each side band.  It all adds up.  fc

Chas, it is simple:

output = carrier X (1/2 + mudulation)
Use Fourier transformation, and you'll get your carrier and sidebands.
Why not zero? Because not exactly 100%. Period.

The varying instantaneous amplitude model, and the three-vector model, are two ways of looking at an amplitude modulated carrier modulated by a sine wave signal.  This signal has three frequency components that beat against each other.  It looks like a carrier varying in amplitude, and in fact that is how we usually synthesize it.  But it also looks like three discrete frequencies of different frequencies in a certain phase and time relationship.

If you had two unchanging carriers of the same amplitude and slightly different frequencies, the amplitude of their sum would vary up and down, from two times the amplitude of one of them, to zero, and so on.  It would have to.   The fact that they occasionally cancel is incidental.  Each is still always present.

In a sine-wave modulated AM signal, the carrier is joined by two slightly different frequencies, spaced exactly the same frequency distance from the carrier, but in opposite directions. One is x higher than the carrier frequency, and the other is x lower than the carrier frequency.  The frequency difference causes a beating - that is, the signals alternately add to and subtract from each other.  The timings are such that when the two side frequencies are in phase (0 degrees relative phase), they are either exactly in phase with the carrier (0 degrees relative phase), or exactly out of phase with the carrier (180 degree relative phase).  This is significant, because the main difference between AM and narrowband Phase Modulation is that in PM, when the two side frequencies are in phase, they are plus or minus 90 degrees to the carrier.

It is not intuitive that the varying amplitude composite signal should consist of a constant level carrier and two side frequencies, yet this is definitely the case.  In an amplitude modulated signal with 100% sine wave modulation, the carrier can be constant even though the instantaneous amplitude of the composite signal might vary from 2x carrier to 0.  The variations are the result of a complex beating between the steady carrier and the steady side frequencies.  In fact with more sophisticated modulator circuits, it is possible to go below zero into the negative!

Going back to PM, even with small side frequency amplitudes, the carrier would still vary slightly in amplitude because of the beating effect.  This is why in PM there are higher order side frequencies generated with sine-wave modulation.  These higher order side currents add to the complex beat pattern in such a way as to eliminate the amplitude variation.  The resulting spectrum is quite complex.

(Fixed a typo 1/16/2005 11:53AM)

Chas Said:

Chas ..  It can't and doesn't. The explanation in the ARRL Handbook is correct. The audio voltage developed across the secondary of the modulation transformer does vary between twice the resting plate voltage and zero, under 100% symetrical modulation. A scope will display the amplitude (vertical displacement) of the transmitter output (audio and RF mixed) versus time (horizontal displacement). Looking at that display you can confirm that the carrier is being completely cut off, for an instant, under 100% modulation. That is why you will see the carrier (output) fall to the baseline, at that time.

Remember that the display on the scope shows you the instantaneous amplitude of the tranmitter output, NOT the frequency. The scope display is a mix of the audio and RF (mixed together) at any one instant .. repeated many times across the oscilloscopes screen.

If you wish to sort out the individual components (LSB, carrier and USB) and see their amplitudes individually, you must use a spectrum analyzer, not an oscilloscope.

I sure hope that helps!

--Larry W8ER

From the beginnings of radiotelephony there has been a question whether sidebands exist as physical reality or only in the mathematics of modulation theory.  In the early 20's this was a hotly debated topic, with a noted group of British engineers maintaining that sidebands existed only in the maths, while an equally well-remembered group of American engineers argued that sidebands do, in fact physically exist.

Today, the issue seems settled once and for all.  We can tune our modern-day highly selective receivers through double-sideband and single-sideband voice signals, and tune in upper or lower sideband, and even adjust the selectivity to the point that we can tune in the carrier minus the sidebands.  Nearly everyone accepts the notion that sidebands do indeed exist physically.  But do they?  Maybe it's a matter of how we observe them, and the result is modified by our measuring instruments.  Remember the Heisenberg Uncertainty Principle, which states that you cannot observe something without affecting it to some degree, or more precisely, that it is impossible to know both the position (physical  location) and velocity (speed and direction) of a particle at the same time.

Imagine a cw transmitter equipped with an electronic keyer.  Also imagine that there is no shaping circuitry, so that the carrier is instantly switched between full output and zero output. Such a signal can be expected to generate extremely broad key clicks above and below the fundamental frequency because of the sharp corners of the keying waveform.  Set the keying speed up to max, and send a series of dits.  If the keyer is adjusted properly, the dits and spaces will be of equal length, identical to a full carrier AM signal 100 percent modulated by a perfect square wave.

Suppose the keyer is adjusted to send, say, 20 dits per second when the "dit" paddle is held down. The result is a 20 hZ square-wave-modulated AM signal.  Now turn the speed up. If the keyer has the capability, run it up to 100 dits per second.  If you tune in the signal using a receiver with very narrow selectivity (100 hZ or less, easily achievable using today's technology), you can actually tune in the carrier, and then as you move the dial slightly you can tune in sideband components 100, 300, 500 hZ, etc. removed from the carrier frequency. A square wave consists of a fundamental frequency plus an infinite series of odd harmonics of diminishing amplitude. Theoretically you would hear carrier components spaced every 200 Hz throughout the spectrum.  In a practical case, due to the finite noise floor, the diminishing amplitude of the sideband components and selectivity of the tuned circuits in the transmitter tank circuit and antenna itself, these sideband components eventually become inaudibly buried in the background noise as the receiver is tuned away from the carrier frequency.

Suppose we gradually slow down the keyer.  As we change to lower keying speed, it takes more and more selectivity to discriminate between carrier and sideband components, as the modulation frequency becomes lower and the sideband components become spaced more closely together. Let's observe what happens when we slow the dit rate down to 10 dits per second. Now the fundamental modulation frequency is 10 Hz, and there are sideband components at 30 Hz, 50 Hz, and 70 Hz removed from the carrier and continuing above and below the carrier frequency at intervals of 20 Hz until the signals disappear into the background noise.  In order to distinguish individual sideband components, we need selectivity on the order of 10 Hz, which is possible if we use resonant i.f. selectivity filters with extremely high "Q".  This can be accomplished using crystal filters, regenerative amplifiers or even conventional L-C tuned circuits if we carefully design the components to have high enough Q.  

As we achieve extreme selectivity with these high Q resonant circuits, we observe a sometimes annoying characteristic familiarly known as "ringing." This ringing effect is due to the "flywheel effect" of a tuned circuit, the same "flywheel effect" that allows a class-C tube type final or class-E solid state final to generate a harmonic-free sinewave rf carrier waveform.  The selective rf tank circuit stores energy which is re-released to fill in missing parts of the sinewave, thus filtering out the harmonics inherent to operation of these classes of amplifier.  CW operators are very aware of the ringing effect of very narrow filters, which can make the dits and dahs of high speed CW run together, causing the signal to be just as difficult to read with the narrow filter in line, as the same CW signal would be if one used a wider filter that admits harmful adjacent channel interference.  Kind of a damned if you do, damned if you don't scenario.
           
Let's now take our example of code speed and selectivity to a degree of absurdity.  We can slow down our keyer to a microscopic fraction of a Hertz, to the point where each dit is six months long, and the space between dits is also six months long.  In effect, we are transmitting an unmodulated carrier for six months, then shutting down the transmitter for six months. But still, this is only a matter of a degree of code speed; the signal waveform is still identical to the AM transmitter tone modulated with a perfect square wave, but whose frequency is one cycle per year, or 3.17 X (10 to the -8) Hz.  That means that in theory, the steady uninterrupted carrier is still being transmitted, along with a series of sideband components spaced every 6.34 X (10 to the -8) Hz.

Now, carriers spaced every 6.34 X (10 to the -8) Hz apart are inarguably VERY close together, to the point that building a filter capable of separating them would be of complexity on the order of a successful expedition to Mars, but it is still theoretically possible. Let us assume we are able to build such a filter.  We would undoubtedly have to resort to superconductivity in the tuned circuits, requiring components cooled to near absolute zero, and thoroughly shield every rf carrying conductor to prevent radiation loss, but here we are talking about something hypothetical, without the practical restraints of cost, construction time and availability of material.  Anyway, let us just assume we were able to successfully build the required selectivity filter.

The receiver would indeed be able to discriminate between sidebands and carrier of the one cycle/year or 3.17 X (10 to the -8) Hz modulated AM signal, identical to a CW transmitter with carrier on for six months and off for six months.  So how can we detect a steady carrier while the transmitter is shut off for six months?  The answer lies in our receiver.  In order to achieve high enough selectivity to separate carrier and sideband components at such a low modulating frequency and close spacing, the Q of the tuned circuit would have to be so high that the flywheel effect, or ringing of the filter, would maintain the missing rf carrier during the six-month key-up period.

This takes us back to the longstanding debate over the reality of sidebands.  If we use a wideband receiver such as a crystal set with little or no front-end selectivity, we can indeed think of the AM signal precisely as a steady carrier that varies in amplitude in step with the modulating frequency.  This is physically the case if the total bandwidth of the signal is negligible compared to the selectivity of the receiver.  Once we achieve selectivity of the same order as the bandwidth of the signal, which has been the norm for practical receivers dating from as early as the 1920's up to to the present, reception of the signal behaves according to the principle of a steady carrier with distinctly observable upper and lower sidebands.  The "holes" in the carrier at 100% modulation are inaudible due to the flywheel effect of the tuned circuits, even though the "holes" may be observable on the envelope pattern of the oscilloscope.

An oscilloscope set up for envelope pattern, with the deflection plates coupled directly to a sample of the transmitter's output, is a wideband device much like a crystal set. It allows us to physically observe the AM signal as a carrier of varying amplitude. A spectrum analyser on the other hand, is an instrument of high selectivity, namely a selective receiver programmed to sweep back and forth across a predetermined band of spectrum while visually displaying the amplitude of the signal. It clearly displays distinct upper and lower sidebands with a steady carrier in between.

Furthermore, it has often been observed that the envelope pattern of a signal as displayed from the i.f. of a receiver can be quite different from that of a monitor scope at the transmitter site.  This is yet another example of how the pattern is altered (distorted) by the selective components of the receiver.

In conclusion, there is no correct yes or no answer to the age-old question whether or not sidebands are physical reality, or exist only in the mathematics of modulation theory. It all depends on how you physically observe the signal.  Sidebands physically exist only if you use an instrument selective enough to observe them. Recall the Heisenberg uncertainty Principle.

I've used FFT based analyzers with micro-Hertz, yes, micro-Hertz resolution capability. Performing DFT/FFT analysis on a signal requires only that you collect an appropriately long time sample.

The sidebands exist just as much as the audio you hear coming out of your speaker. Otherwise one could say, you aren't really hearing that audio, it's just the way your are interacting with the signal. The audio doesn't really exist. :tfrag:  :tfrag:

Well there it is, Chas...a fine description of what we get as a result of mixing audio and a carrier.  

Just for fun, I tuned in WLS on my li'l gen coverage receiver, went to sideband mode, and looked at the signal in SpectrumLab.  Sure enough,  the relationship between the carrier and sidebands holds up.  It may be interesting to record WWV and use software to observe the phase of the carrier and those sidebands.

This discussion sheds some light on why a well designed sync detector is a great asset to an AMer's receive gear...The phasing must be considered along with the bandwidth and amplitudes of the incoming signal.


Thanks for posing the query, and 73!

I contend that we are synthesizing a double-sideband with carrier signal when we modulate amplitude by varying PA B+, or by other means.

You can draw a graph of the carrier, upper sideband and lower sideband sine waves (of course you must make the relative timing/phase correct), and add them together, and the resulting composite signal will be identical to the modulated signal we produce with our modulated stage.  Therefore I conclude that they are the same; that the sidebands exist, and that the carrier is continuous, even though the composite signal may vary from 0 to 2x the carrier level.

Chas .. it is interesting .. and when you get something basic like this out to be examined, all of the different theories are fun to look at.

I guess I can't ignore what I see on the scope and that is a complete cutoff of the output (to include carrier)! If you superimpose (or mix) a 1khz tone on top of a 1 mhz carrier, there are going to be several cycles of RF that occur during the time when the audio tone is at or near complete cutoff of the plate supply. No output is seen at that time. To my mind it seems that the carrier is, in fact, missing, missing at an audio rate.  

So, Bacon I am having a hard time understanding your contention that the carrier is always present. I don't see how IT can be .. and we are talking about those teeny tiny slices of time when the final is not supplied with power.

HUZ what hath you to say about this?

--Larry

I say the carrier goes away, just like in Don's square wave example. But what the heck do I know?

Indeed, 'tis a puzzlement.  I can draw three continuous sine waves of apppropriate amplitude, frequency and phase, corresponding to carrier, upper sideband and lower sideband, and I can add them point by point, and I can duplicate a 100% modulated AM signal waveform as we understand it.  I have to believe that this works backwards; if I make this waveform by modulating a carrier in an AM transmitter, then I am synthesizing the same continuous sine waves.  It seems to me that an on-off sequence would only differ from the sine wave modulated condition in the complexity of the sideband spectrum.

But in the real world, sometimes theory does not produce results.  For example, if there was some FM audio subchannel modulating the on-off carrier, I would simply not be able to receive it while the transmitter was off, even if I was positive that the carrier was theoretically still present.  So as Dennis asked on the amradio mailing list - if a tree falls in the forest, and nobody sees it or hears it... did it really fall?

When I am receiving zero, I can only know that I am receiving zero.  And if there is interference,  I may not even be sure I am receiving zero!

And as Don pointed out, there is the question of reality, and the appearance of reality. If I took the time scale to infinity, then if a carrier was EVER transmitted, even for only a second, I would have to say that it existed for all of time.  That's ridiculous!  I only think that because I am integrating over all time, and then generalizing.

I guess it's off when it's off, and it's signalling us when it's on.  So the question becomes, at what time scale do we stop seeing beat notes, and start seeing variations and on-off switching?

And in a parallel universe.................. ................................................... Great topic

It is very interesting to demonstrate it with  rotating vectors, letting one vector represent the carrier, one upper sideband and one lower sideband.  You can demonstrate both amplitude and phase variations in the carrier, duplicate the envelope pattern, and graphically demonstrate the quatrature distortion that exists with carrier and one sideband, and show that the quadrature distortion in the two sidebands cancels out with double sideband, leaving pure amplitude modulation.  You can demonstrate why the envelope of carrier plus one sideband has low distortion at low percentages of modulation, but becomes highly distorted as the percentage of modulation approaches 100%.

I used to do it with pencil and paper.  It would be very interesting to set up the vector diagrams on a computer where you could actually see the vectors rotate instead of having to imagine motion in still images drawn on paper.  Wish I had the computer savvy to do that.

I am convinced that sidebands exist.  But as for whether the carrier goes away at 100% negative modulation...

How's this: a carrier should be considered to exist during the period when it has recently been detected, and we are actually receiving modulation from it.  It should not be considered to exist when it is clearly not being generated.

So in the example of an AM signal at the instant of 100% negative modulation, we should consider the zero to represent zero percent of the carrier, averaged over time during recent history.  We can see that this will result in appropriate demodulation of the actual signal.

But when the AM transmission is over, either after the end of a transmission, or after the end of a broadcast day, or in most cases if the signal fades out due to propagation, etc, we should consider the carrier not to be present.  This gets a little tricky, because we can not really be certain (Don's Heisenberg Uncertainty Principle thought here) that the transmitter is not sending us a very, very long zero.  But we can be pretty sure, pretty quick.

A synchronous detector is a good flywheel that tracks the known carrier frequency and holds the reference for us during zeros and noise bursts.  We set up a synchronous detector to detect the carrier and hold the reference for a short period of time.  This works pretty well for the signals we actually transmit.

Now take the example of a 1 KHz sinewave transmitted as double sideband suppressed carrier.  You get pulses of alternating carrier polarity.  During any given pulse, for about 500 microseconds at a time, you get an AM signal with a carrier, transmitting one half of a sine wave.  When the pulse is over and the signal passes through zero, it goes negative and the next pulse appears - pretty much identical to the first pulse, but with reverse carrier polarity.  You would not know the carrier polarity reversed, except for your flywheel reference.  But demodulating with the flywheel reference gives you the 1 KHz sinewave.  It is evident to the observer that the flywheel reference was correct.

With a very low modulating frequency, a DSBSC signal would just look like a fading carrier.  The transmit frequency stability and the reference flywheel precision would have to be very high to determine that the carrier polarity had reversed.  At some point this becomes irrelevant, because the transmission path varies in length, there is drift and phase noise in TX and RX, and the signal is not received well enough to know for sure that the carrier polarity flipped.  (Uncertainty again.)

And although with DSBSC we keep getting pulses of carrier, they keep reversing in polarity, and on average they balance out to zero.  We accept that the carrier is suppressed, we don't hear a heterodyne where we would usually hear one, etc, yet we see the carrier dancing on the oscilloscope.  But over the appropriate integration time, its frequent polarity reversals cause it to balance out to zero.

For most real signals, a very long time base is inappropriate.  In some cases though, such as slow synchronous CW, a long time base is appropriate.  So the receiver time scale should be appropriate to the signal being sought.

So it's a reality-check issue.  Surely a carrier was not transmitted for all time, just because it existed for less than a second at some point.  But just as surely, the instant of 100% negative modulation should not be reproduced as a glitch.  The application of the appropriate time scale is the key, and it is up to the listener to determine what happened.

  Bacon, WA3WDR

A class C final and modulator is just a single balanced mixer, that simple.

Actually, isn't it an unbalanced mixer?   Neither signal (rf carrier or modulating audio) is nulled out.  When modulation exceeds 100% the rf output cuts off, and splatter is produced.  Audio is prevented from appearing at the antenna terminals of the transmitter by the selectivity of the rf tank cincuit.

A singly balanced mixer (balanced modulator) would generate double sideband, independent of carrier.  That is what the "upside down tube" circuit does.  Beyond 100% modulation, it continues to produce rf output, but with a 180 degree phase reversal.  It results in distortion with envelope detector, but (theoretically) no splatter.  Cut off the DC voltage altogether, and you have DSB suppressed carrier.  

A doubly balanced mixer is designed to null out both of the original signals so that only the sum and difference frequencies can appear at the output, theoretically without the need for tuned circuits.  There is no need for this in an AM transmitter because two frequencies (rf and audio) are so far removed.  The doubly balanced mixer is useful in rf mixer applications where the two frequencies are close enough together that both need to be nulled out so that they don't appear in the output, such a heterodyne type exciter, typical of SSB rigs.  For example, a 9 mHz xtal oscillator output is mixed with a 5.0-5.5 mHz VFO to generate SSB on 75 and 20 by tuning the output to the sum and difference frequencies.  The DBM makes it easier to eliminate throughput, avoiding undesired radiation of the 5 mHz vfo signal or the 9 mHz output from the SSB generator.

Here's a  phasor representation. Note the display is effectively "strobed" at the carrier rate, otherwise the carrier phasor would be rotating too. [Key: red - carrier, yellow - one sideband, green - the other sideband]. One of these days I will add an envelope display and show a time reference so you can correlate the phasor display to the proper point on the envelope display.

(http://www.amwindow.org/misc/animphasor.gif)

Now we need someone to add a scope and spectrum analyzer display.
Then add a feature to slow things down.
It could be saved somewhere so everytime this comes up the person could be directed to page 53 to see the movie.

Sounds like a job for our smart guy KE1GF

The "resultant" vector, along the Y axis represents the instantaneous amplitude of the output.  The X axis component represents phase (deviation from the strobed carrier position).  Eliminate one of the sidebands and you can see that as the remaining sideband vector rotates, the  resultant vector wobbles from  left to  right as it varies in length (amplitude).  This shows  that  SSB is a combination of amplitude and phase modulation.  In DSB, the phase component of each sideband is equal and opposite;  therefore they cancel out,  resulting in purely amplitude variations.

Here are the phasors with the envelope display included.

(http://www.amwindow.org/misc/phwenvanim.gif)

That is the basis of the "phasing" method of SSB generation - but for that, balanced modulators are normally used, and the carrier is suppressed.

Using the controls and sliders at the link below, you should be able to slow things down, reverse, etc.  No spectrum display though.

http://www.amwindow.org/misc/phwenvanim.mov

TO start, we eliminate magic.

Fact is, in DSB AM, that we all know and love, the carrier is ever present.

It seems that the confusion arises from the method of observation. The oscilloscope displays the algebraic sum of the signals present. The mixer, or modulator is in actuality a multiplier. In simple terms, it mulriplies two sinusoids to produce a product. Supose the carrier signal is represented by Sin(2*PI*Fc*t), where Fc is the carrier frequency, t is time and 2 *PI is necessary to conver the frequency in Hz to Radians per second.  The result is that over the course of one period = 1/t, the argument of the Sine function will go through 360 degrees.  IN a similar fashion, the modulating signal can alsp be a Sine function, Sin(2*PI*Fm*t), where Fm is the modulating frequency.  Sine's sister function Cosine, could also be used as it is Sine shifted by -90 degrees.   In a doubly balanced mixer/modulator, we multply thse two and get upper and lower sidebands...DSB.  For the trig equations for multiply sin and cosine, refer to:

http://mathworld.wolfram.com/ProsthaphaeresisFormulas.html

in particule equations 5 through 8.  

if the balance is upset by adding a constant term (DC) to the modulating signal, the original carrier will be added to the output.

ONe can take independantly generated signals and add them together to get the same result. For example, in the TR-7, AM is generated by first generating SSB and then, prior to the linear PA stage, adding back in the carrier. That rig produces A3H or SSB plus carrier. CHU does this as well.

So, this example of AM being gernerated by adding carrier to a sideband demonstrates it can be done this way.  Clearly, the carrier that is injected after the modulation process is continuous and goes right on out the linear PA section of the rig.

Another thought ot consider. earlier in this thread, a square wave was injected into the conversation as a possible source of modulation, effectively cutting off the carrier during the zero period. Well, I challenge you to examine the square wave itself! IT is agreed to be made up of the fundamental along with an infinite series sum of odd harmionics of decreasing amplitude. Added together, they produce a signal that goes to zero for one half the period. Yet, the individual components are continuous in nature! Observation by oscilloscope would tell us that all the components disappear for one half cycle, just as the carrier appears to  go to zero at the valley of the modulation.  Yet, we know that this is not true.   Pass that square wave through a filter and you can extract whatever component you may want to observe, and it will be continuous.

I can pass the output of my RX's detector through a filter all day long during the period when the carrier is cutoff, and the filter will reveal nothing continuous but noise.

I don't think your TR-7 example is also incorrect. IIRC, the carrier is not linearly added back into the SSB signal, it is mixed/multiplied. And at least one other mixing takes place to shift the IF to the RF output frequency.

Carrier injected after the modulation process...... hmmmmmm
If you add the carrier after the modulation "process"... what was "modulated" ?

Welcome back Buddly! :-P

Thanks Steve !!
Great trip.
Film at 11 !!

Steve,

Thanks for the phasor diagram.  I had been imagining three wheels rotating: LSB, CAR, and USB.  Each one was rotating uniformly faster than the other one, and understanding their phases was driving me nuts!

My wife saw me pointing up, down, and left or right and must've figured this radio theory stuff had gone way too far!  But referencing to the carrier and seeing the other vectors rotating in opposite directions is so much better for digestion...

Now to imagine this combined with Don's 6 month square waves and I'll finally have a grip on the vector model of AM.  To say that modulation is a mixing process has now taken on a more complex meaning.



Y'all have fun with the aurora tonight; I'm pulling the switch early...

Referring to the TR-7, I stand corrected on the details. The carrier is added back in a summing  amplifier after the USB filter, not at the input of the PA.  I was not trying to get into the details of the TR-7, but mearly point out that the carrier can be added back to an SSB signal to create AM.

A simple demonstarion od this principle occurs in radios where the BFO is added to the IF signal prior to detection in a diode AM type detector.  Some others have "talked over" certain braodcasters by using SSB and the other stations carrier.  

Still not convcinced?  Tune in an SSB station with an AM receiver and then add a low level CW  (carrier) signal from a signal generator at the AM receiver's antenna terminal.  You will be able to receive the monkey chatter as if it were an AM signal.

As to the carrier being constant in an AM modulated signal. ponder this: You tune in an AM station with someone in an old buzard monologue.  Someone is tuing up 1 KHz away. You hear the 1 KHz beat note in your speaker. IF the carrier of the station with the talking were not constant, you would not hear a clean continuos beat note.

Get an audio spectrum analyzer and look at a 10 cycle carrier and tell me what you see !

You forgot to consider the time constants of the detector. Unless the carrier was cutoff longer than these, the carrier and any related heterodynes will appear constant.

The beat note of the two carriers, because they are mixing (and therefore intermodulating), is the result of the mixing product "carrier" turning on and off at an audio rate.   Your example thus proves that the carrier does turn off instantaneously.

Or so it seems to me.

73 John

The beat note of the two carriers, because they are mixing (and therefore intermodulating), is the result of the mixing product "carrier" turning on and off at an audio rate.   Your example thus proves that the carrier does turn off instantaneously.

Or so it seems to me.

73 John

I think we can all agree that the plate voltage is zero at 0 % modulation
and 2X plate voltage at 100 % modulation.

Yes !
And what is the power output of a final with 0 plate voltage ?

Uhhh..... 1.21 gigawatts?

1.5 if you are running class e

GFZ said:
Uh ..  did you mean at 100% or are you in standby?

--Larry

Isnt that classE, Larry ?

'Course... let's not mention circulating currents in the output tank !

Eddie Haskell was a cop in  L.A. when I lived there in the '80s
I saw him on TV a couple times.

Does the crank shaft in a motor stop between power strokes?

FC, doesn't zero percent mean no modulation at all and that the plate voltage would be DC plate supply voltage? Isn't 100% defined as being when the plate voltage, under modulation has a crest value of 2 x Vp and a modulation valley value of zero.  That is, of course, unless you live in "The Valley".

OH yeah, forgot your advice about not trying. Apparently, you did too.

I guess your analogy is to the typical Class C RF power amplifier where the conduction angle is notably less than 180 degrees.  So, even though the plate voltage is swinging from 2 x Vp to zero, the RF amplifier tube is only conducting for less that a half RF carrier cycle anyway.  So then I guess it might be more correct to say that the carrier is chopping the audio voltage on the plate of the tube and not that the audio is chopping the RF.  Now, which is the chicken and which is the egg?

In a class E transmitter, the FET switch devicea iare turned on and off by the carrier drive and the drain supply voltage is a audio varying  voltage with a positive DC offset.  Again, the continuous, yet varying, supply voltage is being chopped by the carrier.

Doesn't experience with the production of, and need to filter the odd harminics from a class C output stage agree with this? Then is the typical resonant PI matching network an actual bandpass filter or jsut a ringing circuit. Ah yes, the old flywheel effect, as it was explained when we barely understood algebra, let alone, Fourier analysis.

Yes Rob,
100% negative peak and I like my alge braless myself.
It all just so magic, a modulation transformer and all that.
doin it without a modulation transformer now there may
be a law against that kind of stuff.

Greetings, Gentlemen!

Maybe I can help shed some light on this dilemma.  I think the confusion arises when
considering how the modulated waveform looks in the time domain versus how it looks
in the frequency domain.  When you look at an a.m. signal on a 'scope (time domain)
you see the carrier with the modulating signal swinging the amplitude up to twice the
amplitude of the carrier alone and down to zero at the modulating frequency.  This means
that in the time domain (where the signal is observed as a function of time), the COMPOSITE signal (carrier plus modulation) goes to zero every cycle of the modulating signal.   So, yes, the signal really does go to zero (this means that the
carrier and the sidebands go to zero).  Now, looking at the same sig in the frequency
domain (on your spectrum analyzer) means taking the Fourier transform of the sig, that is, you have integrated over time, so time no longer enters into the picture!  You see the carrier
and two sidebands (keeping it simple with just a sinusoidal modulating signal).  Now you
cannot ask it the carrier or anything else goes to zero at some point in time because time

is no longer a parameter: the time domain picture and the frequency domain picture are
COMPLIMENTARY, with each one giving some information about the sig that the other
cannot give.  That is, if you want to know what the sig does in time, look at it in the time
domain; if you want info about the spectrum, look at it in the frequency domain;  the frequency domain picture is independent of time, that is, to get the freq spectrum you
have to take a "weighted average" of the time-dependent sig over all time (the Fourier
transform), so it is not meaningful to ask what happens at a certain time when looking
at the sig in the frequency domain!  Because the spectrum analyzer does the transform
over and over again at some rate determined by the analyzer you are using, the
spectrum appears to change in time with a complex modulating waveform, but what
you are actually seeing are "snapshots' of the frequency distribution with different modulating waveforms.  So the answer is, YES, the signal really goes to zero with
100% modulation, and YES, the sidebands are real.

Hope this helps some and doesn't increase the confusion!

Here is an illustration.
What do you see on the picture?

(http://wavebourn.com/images/sambr.jpg)

It's a man riding a bycicle and wearing a sambrero hat.

Very well explained! I have always wondered about the "carrier holes" too. Its all about the FLYWHEEL affect! It puts the carrier back in the holes. I know this is an old thread, but it was there so I read it.

You all are making my brain hurt! The picture is a red ball with a drain hole in it hanging along a cable. Side bands exist on paper, and they exist in reality, as proof I offer that I have tuned through them with a narrow filter on the receiver.

Is it wrong to simplify this and say that the upper sideband might be produced by the positive swing of the audio cycle and the lower sideband might be produced by the negative swing, except that the RF frequency is so high that this relationship is reversed every half cycle of the RF wave?

But try doing that with a crystal set.  As far as it, or an oscilloscope with the deflection plates tied directly to the pickup antenna is concerned, it's a single carrier of varying amplitude.


That is absolutely wrong.  The upper and lower sidebands are produced through the sum and difference heterodyning process.  They have nothing to do with positive and negative modulation peaks.

I recall an early issue of QST when slopbucket was first being presented to the mainstream amateur community, in which there was a picture on the front cover that conveyed that same false notion, and as a result I have had people argue with me till they were blue in the face that the diode detector in a receiver converts an AM signal to SSB, because it rectifies half of the envelope of the modulated carrier as it "swings" through the modulation cycle.

If that proposition were true, we wouldn't need those expensive crystal or mechanical selectivity filters to generate SSB.  A simple diode rectifier would do the trick.

Hi


How can the amplifier conti...to deliver the always present carrier....

Maybe because when you modulate the transmitter it's a combination of carrier +modulation with  positive and negative signal .The scope can only "see" the combination of it

When you have a 100 volt carrier modulated it with a sine wave say 50 v Rms,you don't only modulate 50 v negative but also 50 v positive

so  100 v (carrier) -  50 v(negative cycle) + 50 v(positive cycle) = 100 v.

So the carrier amplitude level stay the same seen by the scope.

don't laught at me if I am wrong .Just an idea

Regards


Gito

not the modulation peaks. The modulation, as in the direction and speed a modulating voltage is changing, added to or multiplied with, the direction and speed of change of another voltage the carrier. The modulation voltage is headed + or - at a certain rate, just like the carrier is. The product of the rates of change determine how far out the sideband is from the carrier. It's what I was visualizing with the vectors.

That sounds more like the way I visualise phase modulation that occurs with frequency modulation and vice versa, but I can't see any relationship to LSB and USB with AM.  I would say that both the positive and negative swings of the audio cycle each produce both USB and LSB components, since each of the two sidebands of an AM signal is an exact mirror image of the other.  This is not the case with the sideband components of an FM signal.

HI

Chas stated--  How can------the always present carrier power  when its plate is Zero?

Is it true what Chas wrote?
in the Radio Communication hand Handbook (RSGB )
I have read in Modulation Systems *chapter 9)

.......When the negative peak of the modulation signal has reduced the amplitude of the CARRIER TO ZERO........so no carrier no output I think,and no carrier power output.

That means when we modulate t0 100% ,at peak of negative modulation
the Carrier output is Zero.
but the Modulation proses does not stop there,so we always get a carrier  because the positive and negative modulation,but  in the proses there are times when the Carrier output is Zero ,depending of the amplitude of the negative signal .

Regards

Gito

Its because of the flywheel effect...right? It replaces the carrier back into the voids.

I don't know if it's a flywheel effect but what I know after the negative cycle modulation it's followed  by a positive modulation.
(assuming it"s a sine wave modulation)
The point is if the Plate voltage is zero ,because the negative modulation ,at that time there's no carrier output.

Regards

Gito.

Yes, AM. It's called amplitude modulation of the carrier, from four or greater (depending on voice characteristics) times the peak envelope power of the unmodulated carrier to zero. 

Absolutely, there are times (instantaneously small in a 100%, correctly modulated signal) when the carrier, for that matter the whole signal is zero.

As you exceed, just ever so slightly, 100% modulation of a distortion free sine wave, you'll see zero signal, power, everything.

 The instantaneous crossing into the region of zero power, of course, generates distortion, just like any square wave or very abrubt transistion of current.  Luckily it's at a very low power level. It is an indicator, usually, of much greater distortion in the rest of the signal, given the way such signals are generated by the general population of radio amateurs.  ;D

Is it safe to assume that the actual carrier NEVER goes to zero by offering the example of a SSB station talking to another SSB station on a particular frequency. While listening to them on AM, they would sound scrambled of course. But then when someone else on frequency transmits an AM carrier, you would then hear the SSB stations clearly due to the AM carrier being re-inserted. Would that be proof that the carrier is solid all the time and never goes to zero? It just shows us that on the scope, but the scope shows us an instantaneous display of a frequency even if that frequency is BEATING with another frequency therefore showing us a new strange looking frequency... So when it appears that the carrier is going to zero, it may not be the case in the real world.

Yes, Rick is correct.  The signal goes to zero, carrier included!  Just look at your scope with
an envelope pattern displayed: sig is zero on neg peaks at 100%.  Period.  I think people need to re-read
post #45 from 2005.  I've seen this confusion in courses I have taught, and it seems to arise
from misunderstanding the frequency domain versus time domain.  I tried to explain this in
my previous post, but maybe I didn't do a good enough job!  Try this:  consider a 1 kHz sinusoidal
audio tone.  The amplitude clearly goes through zero every 1/2 cycle, but you don't hear it doing so:
the tone is continuous.  This is the difference between the time domain (sig viewed on o'scope)
and the frequency domain (your ear operates as a detector in the frequency domain, just like
your receiver or spectrum analyzer).  The ear integrates the signal over time, just like the
spectrum analyzer.  After integration, time is no longer a parameter: it has been integrated out
(Fourier transformed out...when you go from time domain to frequency domain, it's a Fourier
transform and the signal has been integrated over time, so time is out of the picture, so to speak,
and any discussion of temporal behavior is now meaningless...you have to work in the time
domain to talk about things that happen in time, such as whether or not the modulated carrier
goes to zero on 100% negative modulation peaks!)

73,
Ken W5KFS

Even though the original question was asked ages ago, and the author has probably moved on to other sites, I'm going to try answering the question that was asked.

The basic question is "why does AM look different when observed from different angles?", the answer is "because you're looking at it from different angles".

When observed mathmatically or through a spectrum analyzer, you see a steady carrier flanked by two opposing sidebands. When observed through an oscilloscope, you see the vector sum of the carrier and sidebands, which appears as a modulated carrier.

The original question asked why you see this modulation on the carrier on a scope and not on a spectrum analyzer. Why don't you see the same unmodulated carrier on a scope that you see on a spectrum analyzer or through mathmatical analysis?

Simple: the scope is not filtered to see only the carrier frequency. It sees the carrier energy and the sideband energy. A heterodyne is a heterodyne no matter where or when it happens, when you combine all these things together, you see a modulated carrier (it works both ways: the carrier heterodyned with the sidebands forms the modulated AM envelope, just as the audio heterodyned with the carrier forms the carrier and sidebands).

The oscilloscope sees the vector sum of the carrier and sideband energy. You're not looking at only the carrier on a scope.

The spectrum analyzer, on the other hand, is very frequency-specific. When it sweeps past the carrier frequency, it (mostly) doesn't detect any of the sideband energy. The carrier itself, without the sidebands to modulate it, appears unmodulated. Mathmatical analysis is even more frequency-specific.

Why doesn't this carrier level shift under modulation during spectral or mathmatical analysis? That too, has a simple answer: because there's no DC component to your voice.

You will see the carrier change amplitude on a spectrum analyzer (or by doing the math) when you observe a modern-day unit generating low-level linear AM and using an ALC designed for SSB. You will see the carrier shrink under modulation, both on a scope and an analyzer. The ALC introduces a DC component that wouldn't be there otherwise.

At any rate, you can't compare scope and analyzer readings. The only thing they have in common is amplitude measurements. Otherwise, they're showing you very different things by very different means. One is a portrait shot, one is a profile shot. One is filtered, one is not.

One is an apple, the other is a dump truck.

You don't see the same thing because you're looking at two different things. Neither piece of equipment is lying, nor is the math. When you consider what is being measured by each unit and how those measurements are taken by each unit, they no longer appear to be mutually-exclusive, and it all makes perfect sense.

I tot that the carrier only disappears in the F.M. mode. Bessel Null (?)
Certain modulating freqs and or amount of deviation result in the disappearance of the carrier.
SSB the carrier is supressed.
PLS clarify me

Fred

Yes, certain modulating frequencies at certain deviation levels cause carrier cancellation in FM transmissions.

The AM carrier does not "disappear". What you see on your scope is a vector summation of the carrier and both sidebands, it is not filtered down to see only the carrier itself.

Its almost as if you need several viewing layers on the scope so that you can view everything seperately without them adding together. Is this possible?

That's called a "waterfall" display. It shows you amplitude vs. frequency vs. time.

An oscilloscope can show you amplitude vs. time. A spectrum analyzer can show you amplitude vs. frequency. A waterfall display shows all three dimensions.

You don't "need" that, though. This isn't some shortcoming of a scope. You need to see your carrier and all modulating energy together to determine if you're modulating properly with a scope. That's the only point to even using one. If you filter your scope down to only view energy within a few hertz (just the carrier, or just a portion of just one sideband), the scope pattern will no longer have any real-world meaning.

Thats it! I need a scope with the waterfall display option. I'll check Ebay. HA...But yes, starting to make some sense now.

HI

Maybe I'm wrong but if I change Chas Question like This 

Can I get an RF output when my final amplifier tube has Zero plate voltage on it?

Can I ?,even with a high power driver?


Regards.


Gito

Answer these three questions:  (1) When the modulating signal swings to it's max negative value,
and the modulation percentage is 100%, what is the instantaneous plate voltage of the modulated
rf amplifier?  (2) What is the instantaneous plate current of the modulated amplifier under these same
conditions?
(3) What is the amplitude of the carrier (output of the modulated amplifier) under these conditions?


I'll answer them for you: (1) zero; (2) zero; (3) zero.

Nuff said.  As my son used to say when he was a toddler, "I done!"

I keep hearing about "instantaneous voltage".

Frequency is a function of time. What happens at a given instant is meaningless without many instants before and after.

You can't do a Fourier transform on a single sample, because there is no frequency information.

At a given frozen instant in time, there is no such thing as radio. Only different voltages on different pieces of matter.

None of this mitigates the fact that the carrier is gone at 100% negative peak modulation.

It doesn't have to. You can't observe an isolated frozen instant of time and still have any concept of frequency.

the modulation domain!!
http://www.home.agilent.com/agilent/product.jspx?nid=-35238.536880491.00&cc=US&lc=eng

I forgot about it. but this discussion's clearing alot of things up.

Geeezy peezy, Talk about making a mountain out of a mole hill! Do any of youse guys walk around with a shirt that says "kick me" on the back? Dont make it any harder than it is.

There is no carrier at 100% negative, 0, nothing, nada! Just look at your monitor scope, it shows a flat line at 100% negative. It shows high peaks (approximately twice the height of the bare carrier) at 100% positive. You dont need $1,000,000 worth of test gear to see this. A simple scope on the output will show this.

You dont need a degree in nuclear physics to see this. And, er, furthermore............
If you cant determine this from looking at your monitor scope you probably shouldn't be operating this type of gear or posess an amateur lisence! !

A final amplifier tube with no plate voltage makes no RF output of its own. It is just that short, sweet, and simple! ! ! ! ! ! ! ! ! !

                                                                 The Slab Bacon

Following that logic, if you can't spell "license", you probably shouldn't have one, either.

So what if your carrier "disappears" for the brief instant you hit 100% negative? That's like saying your carrier "disappears" at the zero crossing: even if it's technically correct, it has no bearing on the practical world.

For that matter, why don't you say that your carrier "disappears" over the 80-some-odd percent of time your grid is driven into the positive, and that the tank circuit doesn't figure into it? That's pretty much the same stance.

But it does have practical application. Overmodulation cuts the carrier on and off, thus creating splatter. You are confusing looking at a particular point on a waveform with not looking at the waveform at all. Instantaneous does not mean you completely disregard what comes before or after. Instananeous voltage and current values are used all the time in tube parameters, amplifier design, breakdown testing, etc. That practical applications are many and varied.

-The Wonderous Quantum nature of electromagnetic transmission. -

The unmodulated AM carrier is contiuous wave RF, hopefully a resonably distortionless sine wave generated by whatever means at a frequency much higher than the modulation frequency.

What your not seeing on a typical waterfall or oscilloscope (unless you speed up the time base a thousand fold or more) is, even on the unmodulated carrier, a sine wave going from so much energy peaking at a "positive" value, through "zero" or no energy and thence to the same peak energy "negative."  Positve and negative are just markers to designate opposite sides of zero.  

Regarding instaneous, we can approach the infinitely small but never achieve it, so instantaneous is defined as the smallest time increment that can be measured, generated, or defined by our sadly limited outlook in the real world.  As instrumentation becomes ever more 'precise' the definition of instantaneous aproaches zero.  But as Steve says the effect is real.  .. the square wave transition from no energy to some finite energy being the summation of an infinite number of sine wave frequencies superimposed. 

We're still talking only the carrier here, or continuous wave.  Don't forget that vigorous electron energy  has "magically"* turned an antenna, say a 1/2 wave dipole into a wave generator of photons carrying energy in both the voltage and magnetic domain.  The wave front radiated, just like the class C tube or whatever generating it goes from positive energy through zero to negative energy as it is propagated outwards in space.  It goes through 'zero' or no energy millions of times a second, say 3,885,000 of cycles per second.

Now when you modulate that CW energy and wave by superimposing other much lower frequency energy, say in the audio range of 'only' 50 to 5000 cycles per second you create an 'envelope' of the previoulsly constant amplitue sine wave RF energy.  Expand the time domain scale so that you can see the individual RF sine waves being modulated by much longer wave audio.  

Your generating energy goes through zero at not only the audio frequecies but thousands of times more at RF frequencies.

Reference superposition theorm and/or algebraic addition in just about any E or EE or physics text.  

* Jury's still out how electrical energy is converted into a wave.  Once you try to capture a photon you collapse the wave front. Squirrly little creatures. Action at a distance faster than light comes into play too.

...to you and I, perhaps; but some of the logic I'm being presented is asking the reader to disregard all else.


I said "practical bearing", not "practical application". There's a huge difference in those two terms.

Yes, fixed numbers are used as limits. What else would we use? Please show me one situation where the disappearance of carrier at that brief instant of 100% negative modulation limits your ability to transmit RF. It doesn't.

Like you said, you cause splatter when you exceed 100% negative, where do you suppose that energy is coming from? Yes, your carrier may be "gone", but you are still transmitting energy with the tube fully cut off.

Zero voltage, zero current, yet greater-than-zero power out of your transmitter, guys! Oopsie! Perhaps focusing on the fact the tube is cut off at a specific instant and ignoring everything else around the tube in time and space is not a scientifically valid approach?

If you choose to freeze time and micro-analyze conditions in a vacuum, you can make almost any argument seem valid.

Actually, that would be 7,770,000 times per second. Every wave has two zero-crossings.

fast paced discussion.
-right.

assumption that 3885 was the fundamental on my oscillator?   ;D

Let's see.
The energy of splatter on the CW zero crossing comes from too much superimposed audio. At the crossing, you've essentially created a square wave with all the consequent infinite number of frequencies.  Once the carrier is zero those spurious freq's and energy dissapates until the audio blows the carrier back up through zero causing another burst of trash.  The length of the zero carrier, cut-off region is a direct function of how much excessive energy audio you have.

thinking some more, uh oh..

The now recovering, overly 'negative' modulated CW waveform coming back up through 'zero' overshoots what would normally be a distortion free 100% modulated signal but it's now coupled with all the square wave trash picked up coming back up through zero carrier.  Superposition again. 

Where did the distortion products go on the down swing through zero in an overmodulated signal?  The were radiated as multiple frequency components and gradually dissapated like any other damped wave, just a lot dirtier.

There's nothing wrong with 'positive' overmodulaton as long as the energy is not allowed to negatively cutoff the carrier.

Right, but I'm talking about something a bit more fundamental.

From the moment you strike 100% negative, you begin to emit square-wave energy (odd-order harmonics, 3f, 5f, 7f, and so on).

At that point, the tube is cut off, and no longer contributing energy.

But that square-wave energy has to come from somewhere. Energy is neither created nor destroyed. Something other than the final has to supply that energy.

That's my point: looking only at the final at a frozen moment of time is just changing the test to match the answers.

The carrier is cut off EVERY TIME the grid swings negative, however that is at a much faster rate than the modulating audio. That is why you have the tank circuit in the final plate line for the "flywheel effect". When your scope has the sweep speed right to display the audio waveform, the sweep speed is way, way to slow to show the individual sine waves from the RF component.

Going back to the flywheel effect, that is why tou have to design the tank circuit differently for a class C amp vs a class AB1 amp etc.

Thom,
        for someone who knows so much, how come you put out such a piss-weak signal?? ??? ???

                                                            The Slab Bacon

You mean the carrier is cut off every time the grid swings positive.


Grow up, Frank. The last time you pulled that one on me, you didn't know I was actually at Tim's place, and strapped you several times during that QSO. That was two years ago, and we haven't worked since then. Go find someone else to crap on.

No energy is emited when the carrier is off. The energy comes from the abrupt turn off and turn on or the sharp rise and fall times. The sharper the rise and fall time, the wider spectral content.

Thom,
         How does the tube cut off when the grid swings positive?? when the grid swings positive the tube is turned full on. When the grid swings negative the electron flow diminishes until you hit the point of full cut off. Therefore increasing the amount of negative biass on the grid diminishes the electron flow, therfore reducing the zero signal plate current.

<snip>
Grow up, Frank. The last time you pulled that one on me, you didn't know I was actually at Tim's place, and strapped you several times during that QSO. That was two years ago, and we haven't worked since then. Go find someone else to crap on.
<snip>

You made the first poke by criticising my spelling. I was merely tossing one back ;D ;D
Would you expect less of me??
It kinda looks like you can dish it out but you cant take it. ;D ;D

And when you are home you are at times rather tough copy down here. I actually do at times have trouble hearing you.

                                                                  the Slab Bacon

Yeah, I got my wires crossed there.

When the grid hits zero, the tube is fully on. When it swings positive, it becomes a second anode and draws power from the tube. So you might say that when you swing positive, you're no longer fully "on", as fewer electrons strike the plate as would with the grid at zero.


It's still a pretty tired line, and it's a claim you make even when I out-strap you. Therefore, I can't rely on signal reports from you. You will always tell me I'm piss-weak.

Yeah, I picked on your spelling, in retaliation for claiming that people don't deserve to have ham licenses because they hold a different view than you do.


I know what to expect out of you, Frank. No suprises there.


I took it just fine, and dished it right back.


I wouldn't know, we haven't spoken in at least two years. The last time I remember working you from home was on my Ranger. That would explain it.

I'm confused... you say no energy is emitted when the carrier is off, then explain where that energy comes from. Is there energy, or isn't there?

Thom,
        It kinda sounds like you need a little refresher course in vacuum tube theory.
When the grid swings positive, it actually acts as an accelerator increasing the electron flow through the tube. Just like the screen grid in a tetrode or pentode.
The current that the grid draws when it is swung positive does not come from the tube itself, but from the preceeding stage. That is why a tube running class B or class C require more driving power than a tube running class A or AB1. The power to swing the grid positive comes from the preceeding stage. Also tubes running A or AB1 are never cut completely off, and never biassed to the point of cutoff. Anyone who has ever messed with big triodes and grid leak bias only (old school) has adjusted the driver circuit and grid tank with the HV to the finals turned off, to be sure he has enough drive to insure he will be able to drive it into class C operation.

<snip>
I'm confused... you say no energy is emitted when the carrier is off, then explain where that energy comes from. Is there energy, or isn't there?
<snip>

There is no output produced at the point of cutoff. It cant be. You still see energy on an analog outpoot meter because the pointer cant respond fast enough.

                                                                 The Slab Bacon

I'm not talking about that. I'm talking about splatter. That happens when the tube is cut off. There has to be a source for that energy. The shape of a waveform is not an energy source, the actual energy radiated as splatter has to come from somewhere.

Sometimes its just better to accept the fact that something is rather than boggling ones brain trying trying to figure out why it exists. I have never really tried to create and examine it, even though I have a spectrum analizer. One day when I nave nothing better to do i will have to create and observe it. However I would rather spend the little free time I have these days doing something more interesting.

My best guess at it would be that the splatter is caused by the stopping and restarting of the flywheel effect of the tank circuit, and / or the voltage transients caused by the shutting off of the voltage at the plate feed caused by negative the modulation peaks being reapplied to modulated final amp. After all these voltage transients when the circuit "unloads" is also notorious for wiping out the insulation in mod transformers.

Sometimes it is just better to accept the fact that something exists than to kill ones self trying to figger out why it exists when you know that it does exist.

                                                                             the Slab Bacon

Strange, that was the exact sentiment that caused me to jump in. We've come full-circle! ;D All the debate of whether sidebands really exist, whether math is real, all spawned from a pretty simple question. I was going to just ask "If W8NB emits a sideband signal and nobody's there to hear it, is he still a pain in the ass?" and let it go, but I thought Chas' original question deserved an answer.

I'm not racking my brain trying to figure it out, I'm just saying that this stuff is never as simple as we all make it sound. There's always a "gotcha" somewhere.

...but yeah, I think we agree that we've gone down this rathole far enough. Passed that point last night some time, actually.

Now just get a real antenna so you can hear me, 'kay?  ;)

Same as "key clicks."  True, the shape of the waveform is not a source of energy, but the rapid rise and fall times imply a broad spectral BW.

Pete

I have heard it argued that overmodulation on negative peaks does not cause splatter because the sharp corner of the waveform as the carrier cuts off in the negative direction occurs only when the rf output from the transmitter has already very closely approached zero.

Boy you sure know how to get us excited! (pun intended)

It could be posited that the sharper the square wave the greater the generation of spurious hash and trash right up through infinity.  Hard to prove otherwise...  I mean a perfect square wave doesn't exist. .. else the whole world would be zapped instantaneously.  ;D

What I'm saying is even a small signal can generate spikes far beyond the much smaller, p.w. CW component right before it hits the zero line....  the sharper the dirtier, with 'spikes' approaching infinity.

I know this is old hat for most of us, but a review of a basic triode Class C amplifier might help.

We will assume only sine wave RF exitation to the grid for 1.9 Mhz and 100 volts Peak or 70.7 V rms, a resonant circuit at the plate tuned to 1.9 mHz with   no modulation , and a 50 ohm load. We will also assume the resonant circuit will filter out any harmonics outside the passband.

The design specs are:
Cutoff bias for the tube is speced at -30 Volts
Plate supply voltage at tube is a well regulated and constant 1000 Volts. Plate current of 100 mA.  Max plate voltage speced at 2,500 Volts.
Grid bias voltage of -90 volts. So the tube is biased three times beyond cutoff.
Grid exitation sine wave voltage has a full cycle of 526 nanoseconds = 360 degrees.

Somewhere on the positive going (first half-cycle) of the grid voltage (total of 263 nS), the grid voltage must rise above -30 volts in order to start turn on. In this case, we drive the grid to a positive +10 volts in order to draw a high peak plate current in a relatively short period of time. So max plate current at 100% is 100 mA without modulation.

At approx. 75 nS into the positive first half-cycle of exitation, grid current and plate current are now flowing and are about 30% Max.

At approx. 132 nS, the RF exitation voltage has peaked to 100 volts and has driven the grid to a full 10 volts positive, so the maximum grid and plate current occur here at 100%.

At approx. 142 nS, the grid and plate current fall to 30% once again.

At about 208 nS, the grid and plate currents have fallen to less than 10%. At 263 nS, all grid and plate currents are zero. The negative half cycle of the RF exitation voltage starts to ramp negatively, but we neglect it since we can't use it.

We have then a plate current pulse of approx. 67 nS duration. This 67 nS (rounded, not rectangular) pulse occurs every 526 nS and shock excites the resonant circuit.

If the Q of the tank circuit is adequate, enough energy will be pulsed into the resonant circuit, such that the resonant (tank) circuit completes a full cycle. If the plate current pulse does not come soon enough (periodically), then the sine wave in the resonant circuit will decay exponentially over time.

If 263 nS represents 180 degrees of the sine wave, then each nS represents about 0.68 degrees. So for this example,  the plate current flows for only 46 degrees of the input sine wave.

An oscilliscope is an amplitude verses time device. On an oscilliscope with suitable probes, one could see the RF input exitation voltage as a complete 200 Volt Peak-to-Peak sine wave, and a plate current pulse of say 100 mA with a duration of 67 nS.

If the overall tube/tank circuit efficiency is 67%, the average power dissipated in the load is 67 Watts. Then with an oscilliscope across the 50 ohm resistive load, one would see a continous sine wave of approx. 164 volts Peak-to-Peak across the load.

On the spectrum analyzer with a probe across the output, one would only see a single spike centered at 1.9 Mhz with no sidebands, since a spectrum analyzer displays dB amplitude verses frequency. With modulation, the trigonometric product of the modulation wave and the carrier wave will show a spike at the carrier frequency with sidebands.

BTW, here is an animated AM modulation site. The modulation frequency is varied and the modulation appears to be a constant 95%:

http://www.williamson-labs.com/480_am.htm

At the risk of being redundant in this long thread…  The power of the RF stage when the scope shows zero is indeed zero. BUT, the energy of the carrier is still a critical component of that zero value.  Were it not for that value of carrier the vector sum of the sidebands alone would yield a very different waveform; namely DSB suppressed carrier. 
Just because the particular instantaneous values of the three waveforms (for a single pure modulating sinusoid) combine at some instant to equal zero DOES NOT infer that their individual values are all zero as well.  Hence the carrier IS actually still there even though the NET output is zero. Just as when crossing zero (twice each cycle), the pure (unmodulated) carrier’s instantaneous value of zero is no less an important component of the waveform as it’s peak value.
I'm sure that clears up everything :) :) LOL

P.S. I think Bacon's first reply said it all as well.

This is how I am understanding this as well. So its not like the carrier is coming and going. It might be a different story if the modulating frequency was the same as the carrier frequency. The signal would then be all screwed up when you said "hello".

Hi

It's true what DMOD wrote about Creating a carrier
but The Created Carrier amplitude(voltage) is depended of the plate voltage Final RF tube,So when the plate voltage is 600 volt you get a higher  amplitude(RF voltage) than 300 v on the plate.
Plate modulation is the way to vary the plate voltage of the Final RF amplifier.
You can manually  " modulate" a RF amply.
With a variable power  supply   ,first we set the plate voltage at 300 volt than look at the carrier amplitude/RFvoltage ,than slowly rise the variable power supply to 600 volt,than slowly make the power supply down to 0 /zero volt.
Than compare the  Amplitude of the Carrier  with 3 different setting Power supply ( 0 volt,300 volt,600 volt)

So it's not how the Carrier is created ,but the Amplitude of the created Carrier with the varying  plate voltage of the Final
 in actual modulation ,the varying Plate voltage of the RF Final is caused by the proses of modulating it.


 A little Question :

If a 10 Hz sine wave  has a 20 RMS voltage we measured with A DC voltmeter, I think the needle of the volt meter is trying to go up and down in phase with the  sine wave,
. its like looking a carrier in a osciloscope ,(WAve -envelope pattern)

If the same sine wave is measured with a AC voltmeter ,we can see a steady voltage on the meter( the needle stay still) is it because it measured the average voltage? why  the voltage does not go up an down like the sine wave?

Because it' design to see the average  amplitude of the wave ! ,rectified filtered with a capacitor than the result is a rectified voltage.that is always there when the sine wave is still on/running.

So the voltage is always present,but actually the voltage going up and down.

Like looking at a spectrum analyzer,we always see a carrier,why? because it is design to measured the average amplitude voltage of the carrier.
actually the Carrier it self is a sine wave which goes up and down.

Regards

Gito.

   When W1ECO and I were kicking around the benefits for 20 Db. of negative feedback in modulation design. One advantage was; When there was an instantaneous higher Z (no reasonable load) from the RF stages the modulator tubes would produce extreme distortion/harmonics which would result in off channel shot noise (not sidebands). But with now more than 20 Db. of negative feedback resulting from less secondary load the modulator gain is momentarily reduced, minimizing perhaps eliminating off channel splatter.

    Therefore I always support a stable 20 Db. mid band gain negative feedback loop. http://hamelectronics.com/k1deu/pages/ham/transmitters/am/pages/negative_feedback_design.htm

    And If we must really process, ego slam the audio the WA1QIX Load device should alsso be added, even to Class E design. http://www.classeradio.com/3-diode.jpg

Regards John

FFT LSMFT and know the answer.
nothing like the high order effects of negative gravity on a hot day.

   Sadly some people still do not understand why Cold fusion only works for those who do Not support War or extreme financial profit !  Regards John

There is cold fusion'; it's just at the quantum level.  Tunnel diode redux.

5 pages of how AM works this must be a new record. Where is Rodney?
I think HUZ covered it about 4 pages ago.

Rodney likes to gab.
I guess we do too.
Fun part of QSO's,
"condoch'aa  row?"  ;D

Well that explains why no one is off the grid with it. In a documentable, provable way, anyway. As soon as the press, gummint, or biz-man with lawyer walks in.. poof!, the stuff must stop working due to their presence. There must be a way to hide bad people from the fusion.

   The Secret of Cold Fusion is known to some but not those who only use their left, very logical brain. 

This is the world of "You can get more out, than what you put in".

To me Science uses both brains combined as one. The Spirit of the Universe, which originates from Billions of billions of stars has many titles/names such as; Zero point energy in Quantum Physics, Prana, even Holy Ghost/Holy Spirit. But no Religion/Dogma or Belief Krap. Only good simple stuff for all to enjoy.

Like it or not this energy only works and supports good Karma. To me this is Science. 73  John, K1DEU